Image sensors are solid state devices used in image acquisition applications. As examples, digital cameras, camcorders and video conferencing machines employ image sensors. Image sensors typically are implemented in an integrated circuit (IC) and include an array of pixels, each pixel being sensitive to received light. A pixel typically produces a voltage which is linearly dependent on the intensity of light it receives. A pixel may be formed of a semiconductor device and particularly from a p/n junction portion of the device. A photocharge generator, within the depletion region of the p/n junction, generates current in response to light received, which current is converted to a voltage.
Shielding light from a portion of the pixels in the array is required for a number of reasons. Typically, several rows and several columns of the array of pixels require shielding. One reason for requiring the shielding is that each pixel from which an output voltage is read requires a dark reference pixel, an output voltage of which is used a dark pixel reference voltage. The signal produced by the dark reference pixel is that ideally produced in the absence of light. Thus, for each row of pixels, there is at least one pixel which requires light shielding so as to act as the dark reference pixel for that row of reference pixels.
Another reason why shielding may be required is that the sensor may be covered by glass and subsequently exposed to light. Circuits peripheral (located adjacent) to the image sensor may be sensitive to light and require operation in the absence of light. As such, those peripheral circuits also require shielding. Further, the semiconductor substrate, on which the pixels are formed, should be shielded from light in order to avoid a photocharge generated on the substrate being collected by the pixels.
Prior art image sensors, often formed as charge-coupled devices (CCDs) using a different process from the standard Complementary Metal-Oxide Semiconductor ("CMOS") processes, typically use a single layer of metal as the light shield. The metal layer is formed to cover the particular rows and columns of the pixel array to be shielded. The quality of the metal shield is important for accurate performance of the image sensor. If the output produced by dark reference pixels is not accurate, which may occur if light reaches the substrate on which the pixel is formed, then the entire row or column, the pixel outputs of which are compared to the dark reference pixel, may produce inaccurate results.
FIG. 1 is a conceptual diagram illustrating a portion of an image sensor including an array 10 of pixels P.sub.n. The array includes twenty-three rows R1-R23 and twenty-three columns C1-C23 of pixels, the pixels being numbered from P1 to P529. Each row Rn of pixels is controlled by row control logic 12. Similarly, each column Cn of pixels is controlled by column control logic 14. Column control logic 14 provides an output to buffer 16. Row control logic 12 and column control logic 14 also may be shielded (not shown).
As shown by shading, the left-most three columns C1-C3 and the upper-most three rows R1-R3 require shielding to provide dark reference pixels for the remaining pixels in those columns and rows.